High-intensity Blue Light as a Night Interruption Can Regulate Flowering of Photoperiodic Ornamentals

Under a short photoperiod, lighting at the end of the day (day extension, DE) or during the middle of the night (night interruption, NI) can regulate flowering of photoperiodic crops. Low-intensity (≈2 µmol∙m⁻²∙s⁻¹) red (R; 600–700 nm) and far-red (FR; 700–800 nm) light controls flowering of a wide range of plants, whereas low-intensity blue (B; 400–500 nm) light generally does not. However, the effects of high-intensity B light, alone or when added to R and FR light, on flowering and photomorphogenesis have not been fully elucidated. We grew five long-day plants [calibrachoa (Calibrachoa × hybrida), coreopsis (Coreopsis grandiflora), petunia (Petunia × hybrida), rudbeckia (Rudbeckia hirta), and snapdragon (Antirrhinum majus)] and two short-day plants [chrysanthemum (Chrysanthemum × morifolium) and marigold (Tagetes erecta)] in a greenhouse at a constant set point of 20 °C under a 9-hour short day with or without 5.5-hour DE and/or 4-hour NI lighting from light-emitting diodes. The B light was delivered at 0, 1, 15, or 30 µmol∙m⁻²∙s⁻¹, in some cases with R + white (W) + FR light at 2 µmol∙m⁻²∙s⁻¹ between 400 and 800 nm. The peak maxima of B, R, and FR light were 450, 666, and 738 nm, respectively. The B light at 30 µmol∙m⁻²∙s⁻¹ created long days in most crops as effectively as R + W + FR light. Flowering of calibrachoa and petunia was 2–4 days earlier, and flowering of chrysanthemum was 11 days later, when B light at 30 µmol∙m⁻²∙s⁻¹ was added to R + W + FR light. For all crops except rudbeckia and marigold, an NI was more effective than a DE. Rudbeckia and chrysanthemum were 14% to 19% and 22% to 36% shorter, respectively, at flowering under B light at 30 µmol∙m⁻²∙s⁻¹ than under mixtures of B and R + W + FR light, but there were few or no height differences among treatments in other crops. We conclude that NI lighting with high-intensity B light, alone and when added to R and FR light, can regulate flowering of various ornamentals.